Quasi-static and dynamic properties of the intervertebral disc: experimental study and model parameter determination for the porcine lumbar motion segment.

PURPOSE The study of axial loading is essential to determine the properties of intervertebral disc. The objectives of this work are (1) to quantify the mechanical properties of porcine lumbar intervertebral discs under static and cyclic compressive loading, and (2) to determine the parameters of a five-parameter rheological model for porcine and compare them with those obtained for human lumbar intervertebral discs. METHODS Thus, the porcine lumbar motion segments were subjected to quasi-static and dynamic compression tests. The quasi-static tests were used to obtain the static stiffness coefficient at different strain rates, while the data from the cyclic compressive tests were used to both determine the dynamic stiffness coefficient and to be fitted in a 5-parameter model, in order to simulate the creep response of the porcine intervertebral discs. RESULTS The results demonstrated that dynamic stiffness coefficient of porcine discs is between four and ten times higher than the static stiffness coefficient, depending on load applied. The parameters of the rheological model suggested a low permeability of nucleus and endplate during the fast response of porcine discs. In addition, the fast response in terms of displacement is four times higher than those documented for human discs. CONCLUSIONS This study revealed that care must be taken on the comparison between porcine and human discs, since they present different behaviour under quasi-static and dynamic compressive loading.